The present invention relates to a method and a system for controlling a vehicle. The invention is further directed to a heavy vehicle and especially to a work vehicle comprising such a control system.
The term heavy vehicle comprises different types of commercial transportation vehicles, such as trucks, buses and work vehicles. The term work vehicle comprises different types of material handling vehicles like construction machines, such as a wheel loader, an articulated hauler, a backhoe loader and a motor grader. A work vehicle is for example utilized for construction and excavation work, in mines etc. Work vehicles are also referred to as work machines. Further terms frequently used for work vehicles are “earth-moving machinery” and “off-road work machines”. The invention will be described below in a case in which it is applied in a wheel loader. This is to be regarded only as an example of a preferred application.
A powertrain is arranged for supplying torque to the wheels of the wheel loader. A previously known powertrain comprises an internal combustion engine (normally a diesel engine), a gearbox and a hydrodynamic torque converter arranged between the engine and the gearbox. The gearbox provides different gear ratios for varying the speed of the vehicle, and for changing between forward and backward driving direction. The gearbox may be an electrically-controlled automatic gearbox of the “power-shift” type.
In addition to supply torque to the wheels, the engine has to supply energy to a hydraulic pump of a hydraulic system of the wheel loader. Such a hydraulic system is used for lifting operations and/or steering the wheel loader. Hydraulic working cylinders are arranged for lifting and lowering a lifting arm unit, on which a work implement in the form of a bucket or other type of attachment or working tool is mounted. By use of another hydraulic working cylinder, the bucket can also be tilted or pivoted. The wheel loader is articulated and further hydraulic cylinders known as steering cylinders are arranged to turn the wheel loader by means of relative movement of a front and rear body part of the wheel loader.
The torque converter is used to increase the torque during particularly heavy working operations, such as filling, the bucket or acceleration of the wheel loader. The torque converter can very quickly adapt the output torque to the current working conditions. However, the torque converter has often a very low efficiency which is also dependent on the current driving conditions. The efficiency can be increased if the torque converter is provided with a lock-up function which can be used for direct operation. However, the gear ratio is fixed (1:1) in the lock-up state, and the problem of low efficiency remains during working operations where such a lock-up function cannot be used.
Further, the torque converter is adapted to protect the engine from sudden rapid changes in the working conditions of the gearbox and the wheels. More specifically, the torque converter provides an elasticity that enables a very quick adaptation of the output torque to the changes in the working conditions of the gearbox and the wheels.
For example, if a wheel loader without the elasticity of a torque converter or similar is driven into an obstacle so that the vehicle stops this will also stop the combustion engine, since the engine in such designs is rigidly and unyieldingly connected to the rotation of the wheels. However, this will not happen if a torque converter or similar is arranged between the engine and the wheels or more preferably between the engine and the gear box. On the contrary, if the wheels of the wheel loader stops this causes the output side (the turbine side) of the torque converter to stop whereas the input side (the pump side) continues to rotate together with the engine. The engine will experience a larger internal resistance from the torque converter but it will not come to a standstill.
Further, the powertrain is individually designed for a specific vehicle type and weight etc. More specifically, the torque converter is individually designed for co-operation with a specific engine and transmission.
In wheel loader operation, the operator controls the engine speed by depressing the gas pedal in order to supply torque for propelling the vehicle and lifting the work implement. The functions propelling the vehicle and lifting the work implement are mutually dependent and the torque converter is correspondingly matched with regard to the design of the powertrain and the hydraulic system. However, due to this design, the vehicle computer cannot know whether the intention of the operator is to lift a load fast or demand a high propelling speed upon depression of the gas pedal.
It is desirable to achieve a method for controlling a vehicle, which creates conditions for a more efficient operation with regard to energy consumption. An aspect of the invention is particularly directed at a method that creates conditions for eliminating a torque converter in the powertrain.
A method according to an aspect of the present invention comprises the steps of receiving an operator input indicative of a desired power for at least one predetermined function, and variably controlling actuation of at least one friction slip clutch in a vehicle powertrain in response to the received operator input in order to control a torque transmitted via the friction slip clutch.
Thus, the friction slip clutch is adapted for torque transmission in a plurality of different actuation states, i.e slip states, and replaces the conventional torque converter in the powertrain. In other words, the friction slip clutch is adapted to maintain a differential speed, i.e a certain slip, during operation of the vehicle, i.e during power transmission.
By means of replacing the conventional torque converter with the friction slip clutch, the low efficiency operation of the torque converter can be eliminated, at least in some operational states.
The friction slip clutch has different inherent characteristics than the torque converter and is preferably controlled, at least in a certain operation state, in such a manner that the characteristics of the conventional torque converter are simulated. The characteristics of the conventional torque converter are for example simulated during load disturbances etc on the powertrain during operation. More specifically, the friction slip clutch is controlled to slip to a certain extent during such disturbances.
Actuation of the friction slip clutch is performed by applying a force on a clutch disc pack, wherein a motive power for propelling the vehicle is effected.
By replacing the torque converter with the friction slip clutch, conditions are created for using one specific friction slip clutch type for a plurality of vehicle types, wherein the friction slip clutch is controlled differently in each individual powertrain type. Thus, conditions are created for an improved standardization. Further, an aspect of the present invention creates conditions for more freedom in powertrain control since the torque transmission via the friction slip clutch is controlled electronically.
Further, an aspect of the invention creates conditions for separating the control of the functions propelling the vehicle and controlling the work hydraulics for lifting etc. This may be achieved in that the propelling speed may be controlled by depression of a gas pedal and that both the speed of movement and the power of the work implement are controlled in response to a position of a manually operated lever. The control unit may be adapted to balance/prioritize the amount of power distributed for propelling the vehicle and the amount of power distributed for controlling the work implement.
According to one embodiment, the method comprises the steps of detecting a vehicle operational parameter, and controlling actuation of the friction slip clutch also in response to the detected vehicle operational parameter. Thus, a torque is determined as a function of the desired power and the vehicle operational parameter. Thereafter, actuation of the friction slip clutch is controlled so that the determined torque is transmitted by the friction slip clutch. Preferably, the method comprises the steps of detecting a vehicle speed and controlling actuation of the friction slip clutch also in response to the detected vehicle speed.
According to a further embodiment, a first operator input is indicative of a desired motive power for propelling the vehicle. The first operator input is received from depression of an accelerator pedal/element. Thus, the torque transmitted via the friction slip clutch is determined on the basis of an input with regard to motive power, and preferably on the basis of depression of the accelerator pedal/element.
According to a further embodiment, the method comprises the step of maintaining a speed of the power source at a substantially constant value at least for one vehicle state. The vehicle state is preferably selected for low gears. Preferably, the engine speed is selected so that the engine supplies a sufficient torque and a turbocharger has a sufficient pressure. Thus, the operator does not influence the engine speed by depressing the gas pedal. Instead, the friction slip clutch is automatically controlled upon depression of the gas pedal. Further, upon a certain depression of the gas pedal, gears are automatically shifted in the gear box.
It is also desirable to achieve a system for controlling a vehicle, which creates conditions for a more efficient operation with regard to energy consumption.
According to an aspect of the present invention, a system is provided comprising an operator controlled element adapted to establish a signal indicative of a desired power for at least one predetermined function, a control unit adapted to receive said signal indicative of the desired power, and in response to said signal establish a signal indicative of a degree of actuation of a friction slip clutch in a powertrain and actuation means for actuating the friction slip clutch in response to the signal from the control unit in order to variably control a torque transmitted via the friction slip clutch.
Further preferred embodiments and advantages will be apparent from the following description and drawings.